Add error messages and unit tests for setups where min_budget >= max_budget

[Bronzila]

Before we simply used an assertion and printed:

Only (Max Budget > Min Budget) is supported for DEHB.

Now we log an error with same message if min_budget <= max_budget, but we also add an additional error if min_budget == max_budget, guiding the user to use plain DE (values are obviously filled with values specified by the user, apart from configspace and target_function):

2023-07-25 16:54:19.802 | ERROR    | src.dehb.optimizers.dehb:__init__:75 - Only (Max Budget > Min Budget) is supported for DEHB.
2023-07-25 16:54:19.803 | ERROR    | src.dehb.optimizers.dehb:__init__:77 - If you have a fixed fidelity, you can instead run DE as follows: AsyncDE(cs=configspace, f=target_function, dimensions=1, pop_size=2, max_age=inf, mutation_factor=0.5, crossover_prob=0.5, strategy=rand1_bin, budget=27, boundary_fix_type=random)

[Neeratyoy]

Looks good. Maybe one more iteration and we can merge.

[Bronzila]

Just checked: both using n_workers and passing a dask Client is safe. I think this can be merged now. @Neeratyoy

[Neeratyoy]

I leave the timing of the branch cleaning part to you for now. Thanks!